The phenomenon of water penetration into sodium octanoate micelles studied by molecular dynamics computer simulation

Critical evaluation of models for self-assembly of short and medium chain-length surfactants in aqueous solutions

During numerous visits to our Laboratory professor Johannes (Hans) Lyklema emphasized the importance of a holistic view on thermodynamics. In order to fulfill this aim he assembled the monumental Fundamentals of Interface and Colloid Science series. The basic state functions (internal energy, enthalpy and free energies) are interrelated by Gibbs and Helmholtz relationships. First-order phase transitions are characterized by first-order state variables (temperature, pressure, entropy, volume). Interactions are, however best expressed by second-order partial derivatives (compressibility, heat capacity and expansivity). They are related to the first-order state variables by relaxation contributions quantifying the degree of cooperativity of self-assembly processes leading to phase separation. In particular they exhibit the limit when phase transitions are changed to second-order processes. This was the focus of my first review dedicated to the memory of professor Lyklema, "Characterization of van der Waals type bimodal,- lambda,- meta- and spinodal phase transitions in liquid mixtures, solid suspensions and thin films" (ACIS 253 (2018) 66). In the present review the attention is placed on short and medium chain-length surfactant self-assembly in aqueous solutions without additives (salts or solubilizates). The dependence of state functions described above on concentration, temperature and pressure is compared to corresponding dynamic molecular processes occurring on different time, frequency and length scales including structure analysis. It is convincingly shown that Hartley-Tanford space filled spherical anhydrous micelle core - polar shell model designed for long chain-length surfactants (cmc < 0.01 mol/dm³, N > 50) cannot be enforced on short and medium chain-length surfactant non-sperical micelles (cmc close to unity, N < 20). Moreover, it is shown that a proper validity evaluation of proposed models for micelle formation is seriously undermined by their application to only a narrow concentration range near critical micelle concentration (cmc). When successful each model should characterize all self-assembly processes occurring (also at limiting association concentration, lac, at second critical concentration, 2cc and at third critical concen-tration, 3cc) within the entire concentration range of thermodynamically stable surfactant solutions. All other self-assembly processes except micelle formation are rarely considered. The pre-micelle formation at lac is, for example omitted as deviations from presented models. The reviewed reports are therefore selected on the basis of maximum investigated concentration range and of largest possible number of homologues.

Phase equilibriums, self-assembly and interactions in two-, three- and four medium-chain length component systems

The Scandinavian surface (surfactant) and colloid science owes much of its success to Per Ekwall and Björn Lindman. In this review the main topics shared by their research groups at Åbo Akademi University in Finland and at Lund University in Sweden are described. The nature of surface active substances (cosolvents, co-surfactants and surfactants) and microemulsions are evaluated. It is shown that the properties of medium-chain length surfactants differ dramatically from long-chain surfactants. The phase equilibriums of binary systems are related to the phase equilibriums of ternary and quaternary systems referred to as microemulsions or more recently also as nanoemulsions. A distinction is made between hydrotrope liquids, detergentless microemulsions, surfactant mixture systems and microemulsions. Three component systems are assembled to "true" quaternary microemulsions. An exceptionally comprehensive network of thermodynamic parameters describing molecular site exchange and micelle formation are derived and related mutually. Gibbs free energy, enthalpy, entropy, volume, heat capacity, expansivity and compressibility can be used to illustrate the degree of aggregation cooperativity and to evaluate whether micelle formation is of a first-, second- or intermediate order phase transition. Theoretical simulations and experimental results show that the associate structures of medium-chain length surfactants are quite open and may be deformed due to small aggregation numbers. The self-assembly occurs over a number of distinct steps at a series of experimentally detectable critical concentrations. Despite the low aggregation tendency their phase behavior equals those of long-chain homologs in surfactant mixture and microemulsion systems. A number of models describing the self-assembly are reviewed. Nuclear magnetic resonance (shift, relaxation rate and diffusion), Laser Raman and infrared spectroscopies were chosen as key instruments for molecular interaction characterization since they were used in the collaboration between the research groups in Åbo and in Lund. A new method is introduced in order to evaluate the traditional procedure for extracting limiting parameters which also enables an illustration of the degree of cooperativity. The focus is laid mainly on aqueous, alcoholic, saline and, to a limited extent oil phases of one-, two-, three- and four component systems of water-sodium carboxylates-alcohol-oil. The extensive thermodynamic characterization of these liquid phases and liquid crystalline phases is left out due to space restrictions.

Modification of the Two-Point Scaling Theory for the Description of the Phase Transition in Solution. Analysis of Sodium Octanoate Aqueous Solutions

On the basis of conventional scaling theory, the two-point scaling theory was modified in order to describe the influence of composition on the partial molar heat capacity and volume during the micellization process. To verify the theory, isobaric heat capacities and densities of aqueous sodium octanoate solutions were measured over wide composition and temperature ranges and the modified approach was used to analyze the calculated partial molar heat capacities and volumes of the surfactant in water. The results obtained indicate that the micellization process is subject to the scaling laws. The results were compared with those for other systems. Peculiar behavior of the critical indices was observed and correlated with the structure of the micelles.

Solute−Solvent Contact by Intermolecular Cross-Relaxation. 2. The Water−Micelle Interface and the Micellar Interior

The intermolecular dipole-dipole cross-relaxation is measured between 19F nuclei of sodium perfluorooctanoate in micelles and 1H nuclei of the water solvent. The cross-relaxation rates for fluorines in the different moieties along the surfactant vary strongly by the resonance frequency in the investigated range of 188-470 MHz. This frequency dependence indicates that the cross-relaxation between water and amphiphilic aggregates is not controlled solely by the fast local water dynamics but significantly contributed to by the long-range translational diffusion of water. The cross-relaxation rates, analyzed in the framework of a model (Nordstierna, L.; Yushmanov, P. V.; Furó, I. J. Chem. Phys. 2006, 125, 074704), provide information about the dynamic retardation of water molecules by the micellar headgroup region and the location of the various moieties along the hydrophobic tail with respect to the water-micelle interface. Both intermolecular cross-relaxation and aggregation-induced 19F chemical shift changes indicate no direct water contact to fluorines except for those closest to the head group.

Molecular dynamics characterization of n-octyl-β-d-glucopyranoside micelle structure in aqueous solution

n-Octyl-beta-D-glucopyranoside (OG) is a non-ionic glycolipid, which is used widely in biotechnical and biochemical applications. All-atom molecular dynamics simulations from two different initial coordinates and velocities in explicit solvent have been performed to characterize the structural behaviour of an OG aggregate at equilibrium conditions. Geometric packing properties determined from the simulations and small angle neutron scattering experiment state that OG micelles are more likely to exist in a non-spherical shape, even at the concentration range near to the critical micelle concentration (0.025 M). Despite few large deviations in the principal moment of inertia ratios, the average micelle shape calculated from both simulations is a prolate ellipsoid. The deviations at these time scales are presumably the temporary shape change of a micelle. However, the size of the micelle and the accessible surface areas were constant during the simulations with the micelle surface being rough and partially elongated. Radial distribution functions computed for the hydroxyl oxygen atoms of an OG show sharper peaks at a minimum van der Waals contact distance than the acetal oxygen, ring oxygen, and anomeric carbon atoms. This result indicates that these atoms are pointed outwards at the hydrophilic/hydrophobic interface, form hydrogen bonds with the water molecules, and thus hydrate the micelle surface effectively.

A Molecular Modeling Study of Pentanol Solubilized in a Sodium Octanoate Micelle

In order to study the structural and dynamical aspects of the solubilization process of pentanol within a sodium octanoate micelle a molecular dynamics simulation is presented. In this initial study we discuss the results and detailed insights into the interactions between sodium octanoate, pentanol, and water. The total micellar radius and the hydrophobic core radius were determined. The calculated values are in fairly good agreement with experimental results. In contrast to pure sodium octanoate micelles the aggregate with dissolved pentanol attained a more spherical shape related to the time interval of the simulation. It is clear that the results of a molecular dynamics computer simulation are always limited by its total length and the total time used for data analysis. Nevertheless, from our simulation study it turned out that a part of the pentanol hydroxyl groups were located within the micellar core and some alcohol molecules were also observed at the surface region of the micelle. The corresponding partition coefficient was calculated and agreed well with the experiment. The evaluated radial distribution functions of the sodium ions, the octanoate oxygens, and the hydroxyl hydrogens reveal details of the interface region of the micelle and the bulk phase. Additionally, it was possible to calculate the trans-to-gauche ratios of the alkyl chains and to compare these results with the simulation of a pure octanoate micelle.